1. Field of the Invention
The invention relates to scanned linear IR detector arrays maintained at cryogenic temperatures and more particularly to the provision of an improved cold shield for such arrays.
2. Prior Art
Effective cold shields are required for infrared detector arrays to limit the unfocused background illumination incident on the array. With line arrays, the simplest approach is to locate the IR array in a cold shielded enclosure with a long narrow slot as an optical entrance aperture to hold the unfocused radiation to a minimum compared with the focused radiation from the scene. When the optics are of low speed, i.e. high F/.music-sharp., this simple approach is very effective. When the optics are required to be "fast", i.e. of low F/.music-sharp., and the detector array is long, the slot must be made wide to match the focused optical cone and then considerable unfocused background illumination can fall on the IR detector, which seriously degrades the signal to background, and accordingly the dynamic range and system sensitivity.
When a fast optical system is employed to obtain high sensitivity, in a scanning infrared sensor it is more difficult to maintain high cold shielding efficiency. Cold shielding efficiency may be defined as the ratio of focused to unfocused radiation incident on the IR detector. The problem posed with wider beam-angle optics is that they force wider and longer cold shield entrance slot and thus expose the IR detector pixels to larger areas of relatively high temperature IR radiating surfaces. Such surfaces include dewar parts, the internal barrels of the optics and to a lesser degree the optical elements themselves. The unfocused background of individual photo detectors in long IR detector arrays, which forces a long entrance slot, can have a background angular field of view approaching a hemisphere in the resolution direction. An ideal cold shield prevents IR background radiation lying outside the focused angular field of view from reaching the individual photodetectors.
Additionally, when a linear array has plural parallel columns causing an increase in the width of the array, the simple slot cold shield admits more unfocused background light in the optical scan direction. A linear array can employ additional columns of pixels in the scan direction for several reasons. Sensor signal/noise can be increased by a time delay and integrate (TDI) operation in which multiple looks from multiple pixels at the same resolution position are combined after suitable time delays to equalize the viewing angle. Another reason is to provide redundant pixels at each resolution position, the best of two or more being selected to overcome bad or poor performance pixels. A third reason is to achieve overlapped pixels at a small pitch with increased resolution by providing additional columns in which the pixels in adjacent columns are displaced in the optical scan direction.
Achieving these objectives has called for additional cold shielding means beyond that available from a simple slotted cold shield. The additional cold shielding has taken the form of cold shields close to the pixels on the detector arrays which affect the viewing angle of either individual pixels or small sets of pixels.
A cold shield which operates at the pixel level for two column staggered element line arrays is described in the U.S. Pat. No. 5,075,553 of M. L. Noble et al filed Oct. 23, 1989. In that patent, a cold shielding arrangement is disclosed for use with either an individual staggered pixel linear IR array or a staggered arrangement of such arrays. The cold shielding arrangement first includes a slotted cold plate. An individual slot is provided for each sub-array on the focal plane, which restricts the field of view of all of the pixels of that array in the optical scanning direction. In addition, a pair of interdigitated cold shields are set on the surface of each array in close proximity to the pixels to restrict the field of view of each individual pixel in the cross optical scan or resolution direction.
The interdigitated pixel level cold shields in the cited patent consist of two comb-like members typically of silicon or alumina formed by micro- machining, one arranged with the frame of the comb arranged outside of one row of pixels and the other arranged with the frame of the comb arranged outside of the second row of pixels. The solution is suitable for two rows of pixels, but is not readily applied to more than two rows of pixels.